ENVIRONMENTAL TOBACCO-SMOKE EXPOSURE AND ISCHEMIC-HEART-DISEASE - AN EVALUATION OF THE EVIDENCE

Objectives: To estimate the risk of ischaemic heart disease caused by exposure to environmental tobacco smoke and to explain why the associa ted excess risk is almost half that of smoking 20 cigarettes per day w hen the exposure is only about 1% that of smoking. Design: Meta-analys is of all 19 acceptable published studies of risk of ischaemic heart d isease in lifelong non-smokers who live with a smoker and in those who live with a non-smoker, five large prospective studies of smoking and ischaemic heart disease, and studies of platelet aggregation and stud ies of diet according to exposure to tobacco smoke. Results: The relat ive risk of ischaemic heart disease associated with exposure to enviro nmental tobacco smoke was 1.30 (95% confidence interval 1.22 to 1.38) at age 65. At the same age the estimated relative risk associated with smoking one cigarette per day was similar (1.39 (1.18 to 1.64)), whil e for 20 per day it was 1.78 (1.31 to 2.44). Two separate analyses ind icated that non-smokers who live with smokers eat a diet that places t hem at a 6% higher risk of ischaemic heart disease, so the direct effe ct of environmental tobacco smoke is to increase risk by 23% (14% to 3 3%), since 1.30/1.06 = 1.23. Platelet aggregation provides a plausible and quantitatively consistent mechanism for the low dose effect. The increase in platelet aggregation produced experimentally by exposure t o environmental tobacco smoke would be expected to have acute effects increasing the risk of ischaemic heart disease by 34%. Conclusion: Bre athing other people's smoke is an important and avoidable cause of isc haemic heart disease, increasing a person's risk by a quarter.